Modern electronic products including, for example, consumer electronics, computers, telecommunication equipment and automobile electronics require electrical device interconnection and packaging. Typically, solder joints are used to interconnect packaged or unpackaged integrated circuit devices to printed circuit boards or substrates. Conventional methods for forming such solder joints include wave soldering or the use of solder paste. Reduction in the size and cost of electronic component has been a general goal of the electronic industry.
However, the reduction in electronic component size generally requires an increase in the density of component interconnections and a reduction in the size and height of corresponding solder joints. Stress cycling due to thermal expansion mismatch between two interconnected components and thermal cycling during the operation of electronic components causes local recrystallization and microstructure coarsening in associated solder joints. This coarsening in combination with the increased shear strain experienced by smaller solder joints increases the likelihood of thermal fatigue crack growth and solder joint failures. Such behavior has been observed with conventional Pb--Sn solders as described in D. R. Frear et al., Solder Mechanics, ch. 6, p. 239 (1990), which is incorporated by reference herein, and has undesirably limited the reduction in solder joint size and corresponding electronic component size.
The previously cited Solder Mechanics book and Inoue et al, "Pb--Sn Solder for Die Bonding of Silicon Chips", IEEE Trans. on Components, Hybrids, and Manufacturing Technol., vol. 9, p. 190-194 (1986), have recognized that solders with fine grains, such as grains having a diameter in the range of 1 to 5 .mu.m are somewhat resistant to coarsening and correspondingly provide a somewhat improvement in the long term reliability of solder joints. However, conventional solders with such fine grains are not sufficiently stable, and still experience undesirable coarsening from thermal and stress cycling during the useable life of typical electronic components.
As a result, a recognized need exists for solder joints that are substantially resistant to coarsening and fatigue from thermal and stress cycling.